This application claims Priority from German Application No. DE 102 17 507.1 filed on Apr. 19, 2002.
The present invention describes an array imaging a pulsed particle beam emitted from at least one point on a sample on to a detector.
It is known that so-called particle lenses, i.e. lenses for charged particles like electrons can influence the paths of these particles in a similar way as an optical lens influences photon rays.
Such particle lenses are applied e.g. in electron microscopes. The chromatic and spherical aberrations of particle lenses are the main factors limiting spatial resolution. Astigmatism is the result of misalignment and can be compensated by electric or magnetic stigmators and correcting image curvature which is often of secondary importance. However, the chromatic and spherical aberrations constitute a basic problem for all optical systems composed of round lenses.
The chromatic aberration is caused by the different energies of the particles in the beam and results in the fact that particles of different energy do not intersect the ideal image point. In other words: due to the chromatic aberration particles with different energy hit the image plane in different positions.
In contrast to photon optics these aberrations cannot be corrected by suitable lens combinations. The reason is that, independent of type and geometry of the selected lens, all particle optical round lenses are characterized by spherical and chromatic aberration coefficients c, and cc, respectively, that are always positive. Thus, the combination of lenses with coefficients with different sign in a lens system of round lenses is impossible.
This fundamental property of all optical round lenses for particles is known as Scherzer""s theorem [O. Scherzer, Z. fxc3xcr Physik 101 (1936) 593].
Scherzer [O. Scherzer, Optik 2, 115 (1947)] and other authors have searched for ways to circumvent this theorem and discussed different possibilities for the correction of c, and cc.
An overview is given by Hawkes and Kasper [Principles of Electron Optics, Academic Press 1996, Vol. 2, p. 857ff]. The conditions for the validity of Scherzer""s theorem are: round lenses, real images, static fields, no space charge, no potential jumps. These conditions imply possibilities to circumvent the theorem. Despite numerous attempts with different methods, only the use of multipole correctors in high-resolution transmission electron microscopes has been successful up to now [M. Haider, S. Uhlemann, B. Schwan, H. Rose, B. Kabius, K. Urban, Nature 392 (1998) 768]. A second possibility of aberration correction by means of electron mirrors was proposed and is being tested [G. F. Rempfer et al., Microsc. Microanal. 3 (1997) 14; R. Fink et al., Journal of El. Spectrosc. Relat. Phenom. 84 (1997) 231]. These methods pose a high challenge on the mechanical precision and the current or voltage stability of the electronics. In addition, the adjustment is very demanding, in particular in the case of the non-linear optical axis connected with the solution of the electron mirror.
Several authors discussed the application of so-called high-frequency lenses [Hawkes and Kasper, Principles of Electron Optics, Academic Press 1996, Vo., 2, p. 872 ff], where the lens is formed by a micro wave resonator or the resonator is integrated into a lens. It turned out that the phase condition (i.e., the relation between the phase of the micro wave and the phase of the electron bunch when entering the resonator) and the dwell time in the resonator plays a crucial role. None of the proposals in connection with high-frequency lenses has been realized successfully.
In reference (U.S. Pat. No. 5,196,708 A) a particle source is described that produces a pulsed particle beam in a way that the particles belonging to a bunch experience a pulsed electrical field in a so-called xe2x80x9cbuncherxe2x80x9d synchronized with the pulses. This pulsed electrical field causes an acceleration of the electrons at the end of the bunch. This acceleration causes a desired shortening of the axial expansion of the particle formation. This axial compression can be used for imaging with reduced chromatic aberration. This energy focussing effect using a pulsed axial electrical field is also known from time-of-flight mass spectroscopy (Rev. Sci. Instr. 26(1955) 12, 1150-7 (Wiley. W.C.: McLaren. I.H.), section on time-lag energy focussing, p. 1154. The reduction of flight-time differences by pulsed fields can also be exploited for an improvement of the mass resolution of a time-of-flight mass spectrometer. (DE 44 42 348 A1). These methods cause a temporal and spatial compression of the particle ensemble at the target (called bunching effect in reference U.S. Pat. No. 5,196,708 A).
The contribution of the chromatic aberration to the total resolution of an optical system is given by
xcex4c=ccxcex1xcex94E/E 
where cc is the chromatic aberration coefficient dependent on lens geometry and energy, xcex1 is the beam pencil angle accepted by the contrast aperture and xcex94E/E is the relative width of the energy distribution of the imaged charged particles with centre of gravity at energy E.
It is an object of the present invention to provide a device that enables correction of the chromatic aberration of a system of round lenses for charged particles, but wherein the complexity of the arrangement is greatly reduced. This is achieved via energy dependent axial expansion, i.e. energetic dispersion, of the pulsed particle beam in a low-energy drift space.
According to a first aspect of the present invention, an arrangement for imaging of a pulsed particle beam from a sample to a detector with minimized chromatic aberration comprises: an imaging system with particle optical round lens optics, a low-energy part of the optics called drift space, means for application of a rapidly switched voltage such that the velocity of the particles of the beam is changed in a way that the particle rays starting in one point on the sample intersect the image plane in one point thus leading to a reduction of the chromatic aberration in the image. To achieve this, the fastest particles reach the end of the drift space before the switching of the electrical field and are thus not influenced by the field. The slower particles, however, are accelerated by the application of a suitable voltage in a way that the energy gain is proportional to the local potential in the moment when the voltage is switched on. Their energy gain is the higher, the slower the particle has been on its way through the round lens optics. This means that the particles at the front of the beam retain their energy, whereas the particles in the centre of the beam gain a higher kinetic energy and the particles at the end of the beam gain the highest kinetic energy.
The energy distribution of the particle beam can be inverted if a parabolic potential is applied. As the originally fastest particles retain their velocity or energy, however the slowest particles are also accelerated to a new energy higher than the originally fastest particles, the chromatic aberration of the following round lens optic can compensate the chromatic aberration of the preceding part of the optics. The particles are intersecting in the same image point, i.e. the total chromatic aberration vanishes in the sense that all particle rays intersect in the ideal image point. This implies that the voltage has to be switched in the moment when the particle beam is in the accelerator at the end of the drift space. Further advantages are described in the claims.
According to a special embodiment of this first aspect of the invention, the particles in a particle beam can be decelerated by a retarding voltage and dispersed in a subsequent low-energy drift space such that the distance between particles with different energies is increased. This deceleration of the particles by means of the to retarding voltage retains the velocity differences between the different particles but reduces the average velocity of the beam. Hence, electrons with different velocities gain distance from each other. Behind the drift space the particles experience an energy increase in the pulsed accelerator such that the slower particles gain more velocity than the faster ones. Consequently, all particle rays intersect in the ideal image point leading to a vanishing chromatic aberration as described above.
According to a second aspect of the present invention, an arrangement for imaging of a pulsed particle beam from at least one point of several points on a sample comprises: an imaging system with particle optical round lens optics, a low-energy drift space increasing the spatial and temporal distance between the particles of the particle ensemble with different velocities such that after passing of the particles through the drift space the refractive power of the round lens optics behind the drift space is adapted at each moment such that the focal width of the optics remains the same for all particle energies and such that all particle energies of the beam are imaged with the same magnification on the screen. Consequently, the chromatic aberration is eliminated in the image.
Further advantages of this second aspect of the present invention are described in the claims.
The visualization of the particle-optical image requires a detector. Such a detector can comprise a fluorescent screen and a CCD-camera that acquires the image. Alternatively, the detector can be designed such that all particles are individually detected and registered with respect to their spatial coordinate and arrival time on the detector. This bears a number of advantages: it provides the mode of energy-resolved imaging of the particles or the possibility of imaging of selected particle energies such that the partial images corresponding to different energies can be adjusted in scale resulting in a sharp image. The adjustment of the partial images taken at different energies to the same scale requires compression, expansion, rotation or displacement of each partial image by means of suitable image acquisition software. Furthermore, this procedure allows compensating the influence of spurious stray fields.
Of particular advantage is the application of both aspects of the present patent in an arrangement imaging pulsed particle beams emitted from a sample in an electron or an ion microscope or in optics for projection lithography. It can also be exploited in the generation of pulsed electron or ion microprobes.